The mechanism of cell fusion in the unicellular biflagellate green alga Chlamydomonas Reinhardi will be studied. The proposed approach will include experiments designed to characterize fusion organelles present on both mating type plus (mt plus) and mt- gametes. Experiments will seek to characterize purified mating structures isolated from mt plus and mt- gametes; emphasis in these experiments will be placed on changes related to activation of the mating structure for cell fusion. Activation in the mt plus gamete involves the polymerization of microfilaments into a fertilization tubule. We plan to investigate whether these filaments are similar to actin by heavy meromyosin labeling and detection of 3-methyl histidine. We also plan to examine the mating structure in Imp-1 a mutant with a defective mating structure. Once we have devised a method for mt plus mating structure isolation we propose to concentrate attention on mt- mating structure. Activation of the mt- structure involves membrane particle aggregation. We plan to cross-link membrane particles and carry out isolation experiments. Further, fluorescent labeling with selected lectins will also be used to study changes in the outer surface after mating structure activation. We will also examine factors controlling membrane particle aggregation in mt- mutants we propose to isolate. This should allow us to clarify some of the interactions which control intramembranous particle aggregation.